Deposition of silver layer on nonconducting substrate

ABSTRACT

A thin silver layer is deposited on the surface of a nonconducting substrate using an electroless process. The surface is cleaned and activated in aqueous solution containing stannous tin. The silver is deposited as a colloidal material from an aqueous solution of a silver-containing salt in the absence of an electric current, but in the presence of a deposition control agent. Optionally, the silver layer is stabilized with an aqueous solution of a platinum-group metal and/or gold. The resulting silver layer is uniform and of 2-2000 Å thickness; it strongly adheres to the substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Rule 60 continuation of application Ser. No.07/897,614 filed 10 Jun. 1992, now U.S. Pat. No. 5,395,651, which iscontinuation of Ser. No. 630,333 filed 13 Dec. 1990, now abandoned whichis a continuation of Ser. No. 347,016 filed 4 May 1989 and abandoned.Also related is U.S. Pat. No. 5,320,908 issued on Ser. No. 17,623 filed12 Feb. 1993 which is a division of the parent application herein, Ser.No. 897,614.

BACKGROUND OF THE INVENTION

This invention relates generally to the deposition of silver films onnonconducting substrates, and, more particularly, to deposition of suchfilms that are very thin.

Thin films of certain metals on nonconducting substrates can haveimportant commercial applications. Thin films of conducting metals ontransparent substrates are used in electronic display devices. Thinfilms can be used to reflect heat in solar shading or other solardevices, and to filter radiation from sunlight. A thin film can reducethe incidence of infection caused by a device that is introduced intothe human body, when the film is coated onto the device beforeintroduction into the body. Thin films are used in packaging as a vaporbarrier coating. These applications are only illustrative of thethousands of uses of thin films, and are not limiting of their uses.

In one particular application, films of silver or silver-containingcompounds are particularly effective in reducing microbial infection inthe human body. U.S. Pat. No. 4,404,197 describes the use ofsilver-containing salts in reducing the likelihood of infection of burnvictims. U.S. Pat. No. 4,581,028 describes the use of antimicrobialsilver salts in implants, and U.S. Pat. No. 4,603,152 describes othersuch devices utilizing silver compounds to resist infection. U.S. Pat.Nos. 4,054,139 and 4,483,688 disclose the use of a pure silver metalliccoating on medical devices to reduce the incidence of infection. Thus,it is well established that coatings of silver or silver compounds areeffective in reducing the chances of infection caused by medical devicesthat are implanted or inserted into the body.

Although the value of using silver to avoid infection is wellestablished, there is less knowledge as to effective approaches to thebest approach to providing the silver on the surface. Electrodepositionmight be used, but in most cases the medical instruments are made ofnonconducting materials which cannot be readily coated electrolyticallyon a commercial scale, with a thin, adherent coating. The '139 patentsuggests that coating "of the type deposited by electroless deposition"would be operable, but gives no details of operable processes. The '688patent describes the use of large 300 mesh particulate silver to coatcatheters.

Silver can be coated onto nonconducting substrates by electrolessprocesses. One example is the process used to coat silver onto mirrors,but such coatings are comparatively thick. Another example is drydeposition techniques such as vapor deposition or sputtering, but thesecannot be used to coat irregularly shaped substrates, or the insides oflong bores.

Because silver can be toxic in some circumstances, and is expensive, itis preferable to coat the silver as a very fine layer onto theelectrically nonconducting substrate. The coating should be stronglyadherent to the substrate, because loss of the coating might result ininfection or passage of the silver into the body. There are notpresently known techniques for depositing silver onto various types ofnonconducting substrates that permit the deposition of a very thin, butuniform, transparent layer of silver, on the order of 2 to 2000Angstroms thick, produce a highly adherent layer with good mechanicalproperties, and are readily adapted to large scale commercialmanufacturing of coated products.

There therefore exists a need for such a coating technology. The presentinvention fulfills this need, and further provides related advantages.

SUMMARY OF THE INVENTION

The present invention provides a process for depositing thin, uniformlayers of silver onto a wide variety of nonconducting substrates. Thesilver layer is adherent and effective in various uses, including, forexample, antimicrobial medical applications, barrier packaging, andoptical filters. The process can be performed at ambient temperature or,at most, slightly elevated temperature, using conventional industrialchemical procedures. It is highly controllable and reproducible,producing virtually identical layers on large numbers of substrates.Tests have shown that the yields of good quality coated parts using theapproach of the invention is very high.

In accordance with the invention, a process for depositing a uniformthin layer of silver onto the surface of an electrically non-conductingsubstrate comprises the steps of activating the surface in an aqueousactivating solution containing at least about 0.001 grams per liter of asalt containing stannous tin ions; and depositing silver onto thesurface from a deposition solution of a silver-containing salt, areduction agent in a concentration sufficient to reduce the silver saltto form metallic silver at the surface of the substrate, and adeposition control agent in a concentration sufficient to prevent thesilver in the solution from precipitating from the solution, and topermit it to deposit upon the surface of the substrate,the step ofdepositing being accomplished in darkness. If necessary, the surface ofthe substrate may be cleaned prior to processing. Preferably, the silverlayer is stabilized after deposition, but before use.

The preferred approach deposits a thin, uniform layer of silver,preferably 2 to 2000 Angstroms thick, at the rate of about 5-7 Angstromsper second in the deposition solution. The thickness of the surfacelayer is readily controlled. The resulting silver layer is adherent tothe surface of the nonconducting substrate. Other features andadvantages will be apparent from the following more detailed descriptionof the preferred embodiments, which illustrate, by way of example, theprinciples of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In accordance with a preferred aspect of the invention, a process fordepositing a uniform thin layer of silver onto the surface of anelectrically non-conducting substrate comprises the steps of cleaningthe surface in an aqueous cleaning solution; activating the surface inan aqueous activating solution containing at least about 0.001 grams perliter of a salt containing stannous tin ions; depositing silver onto thesurface from a deposition solution having a pH of not less than about 8,and containing silver nitrate, a reduction agent selected from the groupconsisting of formaldehyde, hydrazine sulfate, hydrazine hydroxide, andhypophosphoric acid, in a concentration sufficient to reduce the silversalt to form metallic silver at the surface of the substrate, and adeposition control agent selected from the group consisting ofinvertose, succinate acid, sodium citrate, sodium acetate, sodiumhydroxide, potassium hydroxide, and ammonia in a concentrationsufficient to prevent the silver in the solution from precipitating fromthe solution, and to permit it to deposit upon the surface of thesubstrate, the step of depositing being accomplished in darkness; andstabilizing the deposited silver by contacting the surface upon whichcolloidal silver has deposited for at least about 5 seconds with astabilizing solution of at least about 0.001 grams per liter of a saltof a metal selected from a member of the platinum group and golddissolved in dilute nitric acid, the resulting solution having a pHvalue of from about 3.0 to about 4.8.

The present invention is operable in depositing a colloidal metallicsilver layer upon the surfaces of many different nonconducting substratematerials. The substrate may itself be a flexible film, or may be arigid solid. Such materials include, by way of example and notlimitation, latex, polystyrene, polyester, polyvinylchloride,polyurethane, ABS polymers, ceramics such as aluminum oxide, glass,polyamide, polyimide, polycarbonate, and synthetic rubber. The nature ofthe surfaces of these materials varies widely, but the present approachis applicable for all.

It is important that the surface of the substrate be sufficiently cleanthat it can be wetted by subsequent activation, deposition, andstabilization solutions. Contaminant layers of grease, oil, dirt,chemicals, and other materials can interfere with the ability of thesesolutions to react with the surface. If the surface is sufficientlyclean initially so that wetting can be accomplished, no further cleaningis necessary. However, for many nonconducting substrate materials incommercial applications, cleaning is necessary because the surface hasnot been sufficiently protected from dirt and organics prior to thesilver deposition operation. The surface of such a substrate ispreferably cleaned by a technique appropriate to that particularnonconducting material.

For example, polycarbonate, polyamide, polyvinylchloride, polyurethane,and polyester may be cleaned in a 5 percent sodium hydroxide solution at40° C. for 10 minutes. Polystyrene may be cleaned in a 10 percent sodiumhydroxide solution at 30°-40° C. for 5-20 minutes. Latex and syntheticrubber are cleaned in a 3 percent sodium hypochlorite solution atambient temperature for 1-5 minutes. Ceramics such as aluminum oxide maybe cleaned in a 25 percent sulfuric acid solution at 60° C. for 20minutes, with at least about 5 minutes including simultaneous ultrasonicagitation. Polyimide film may be washed in acetone. Glass is cleaned inan aqueous solution of 0.5 percent hydrofluoric acid and 10 percentsulfuric acid, which imparts a slight etch to the glass. ABS polymer maybe cleaned in an aqueous solution containing 350 to 400 grams per literof chromic acid, and 25 percent sulfuric acid, at 65°-70° C., for 5 to10 minutes. These cleaning treatments are illustrative and not limiting.

The cleaning, if necessary, is normally accomplished by immersion of thesubstrate into the cleaning solution, but the cleaner may be sprayed,brushed, or otherwise applied to the surface. If cleaning is required,the surface is thoroughly rinsed in demineralized water after cleaning,but the surface is not dried. At several points in the process aspreferably practiced, the substrate surface is rinsed in demineralized(or deionized) water. It is important that this be a thorough rinse,because chemicals transferred from one process step to another mayinterfere with the subsequent step.

The clean surface of the substrate is activated, also termed sensitized,to prepare it for the deposition step. Activation is accomplished in adilute activation solution containing at least 0.001, preferably 0.01 to0.5, most preferably 0.01 to 0.2, grams per liter of a salt containingstannous tin ions. The adjective "stannous" indicates that the tin ionsof the salt are in the +2 or (II) oxidation state. Preferred salts arestannous chloride (SnCl₂) and stannous fluoride (SnFl₂). The selectedsalt is dissolved in acidified demineralized water to form theactivation solution. The pH of the solution is preferably from about 1.2to about 3.5, most preferably about 2.5, attained by adding the requiredamount of hydrochloric acid. The activation solution is preferablyfreshly prepared, not stored for more than about 1 day, although thelife of the solution is longer for more dilute concentrations.

Treatment of the surface in the activation solution is preferably forabout 5-30 minutes at ambient temperature. After the treatment iscomplete, the surface is removed from the solution and rinsed thoroughlyin demineralized water, but not dried.

The activated and rinsed substrate is transferred to the depositionsolution. The transfer is preferably done immediately, but tests haveshown that the activated substrate may be stored in demineralized waterfor at least several days. The silver deposition solution is preferablyfreshly prepared, no more than about four hours prior to use, and has apH of not less than 8. The deposition solution is preferably not usedfor too many substrates, as the quality of the deposited film can bereduced if the solution is used too many times. It includes asilver-containing salt, preferably silver nitrate (AgNO₃), in aneffective amount of no more than about 0.10 grams per liter, preferablyabout 0.015, grams per liter. If the silver content is above about 0.10grams per liter, the elemental silver may form nonuniformly, in thesolution or on the container walls. Expensive silver may be wasted,because the deposition solution is preferably discarded after 2-3 uses.If the silver content is below an effective amount, there isinsufficient silver to form a film in the desired time.

A second component of the deposition solution is a reduction agent thatreduces the silver-containing salt to elemental silver. The reductionagent must be present in an amount sufficient to accomplish thatchemical reduction. Acceptable reduction agents include formaldehyde,hydrazine sulfate, hydrazine hydroxide, and hypophosphoric acid. It ispreferably present in an amount of about 0.001 milliliters per liter ofsolution. Too large a concentration of the reduction agent causesdeposition of silver throughout the solution and on the container walls,while too small a concentration may result in an insufficient formationof metallic silver on the substrate.

Another component of the deposition solution is a deposition controlagent that is present in an amount sufficient to slow the depositionreaction to prevent the reduced metallic silver from precipitatingdirectly from solution as a fine metallic powder, or precipitating ontothe walls of the container. Operable deposition control agents includeinverted sugar, also known as invertose, succinate acid, sodium citrate,sodium acetate, sodium hydroxide, potassium hydroxide, and ammonia. Thedeposition control agent is preferably present in an amount of about0.05 grams per liter of solution. If too little is present, theabove-described precipitation from solution of metallic silver particlesmay occur. If too much is present, the silver-containing salt may becometoo stable for the desired precipitation onto the substrate of interest.

The concentrations of the reduction agent and the deposition controlagent may be adjusted as necessary to achieve the desired results,depending upon the substrate material, the thickness of the filmdesired, the conditions of deposition, and the concentration of silverin the solution. For example, for thin films the silver saltconcentration will be relatively low, as will the concentrations of thereduction agent and the deposition control agent.

In preparing the deposition solution, each of the components of thesolution is preferably individually dissolved in demineralized water.The various pre-solutions are then mixed, and diluted where necessary,in the correct amounts to achieve the concentrations indicatedpreviously. Mixing the components together during the solution-formingstage may result in instability and precipitation of silver prematurely.If the solution is to be stored before use, it must be stored indarkness to prevent undesired deposition.

The silver salt that is the source of the deposited silver is highlysensitive to decomposition by light in the visible range, and such lightis therefore excluded from the deposition procedure. The combination ofsilver salt and reduction agent, used in darkness, permits the silver tobe reduced from the salt in a colloidal state to be deposited upon thesurface of the substrate. This colloidal state is particularlybeneficial to achieve good adhesion of the completed silver film to thesubstrate surface, good transparency as a thin film, biocompatibility,tissue friendliness, and non-toxicity. Various of these properties maybe important in different applications of the thin film. Good adhesionis important in nearly all uses. Biocompatibility, tissue friendliness,and non-toxicity are particularly important in medical applications.Uniform transparency is critical for electrical instrument requirements.

The substrate surface is exposed to the deposition solution by anyappropriate procedure. Dipping into the solution is normally preferred,but the solution may be applied by any convenient technique such asspraying or brushing. The silver film deposits uniformly from thesolution at a rate that may be controlled by the concentration of thesilver salt. With a concentration of about 0.015 grams per liter ofsilver nitrate, the deposition rate is about 5 Angstroms per second atambient temperature, although in some circumstances the rate may be ashigh as about 7 Angstroms per second at ambient temperature, with thedeposition rate increasing with increasing temperature. If a thin filmis required, the temperature of deposition is maintained sufficientlylow that deposition is controllably slow. Thus, a repeatable, uniformthin film about 50 Angstroms thick may be prepared by immersion for 10seconds. Increasing the deposition time increases the film thicknessproportionately, at least up to thicknesses of about 2000 Angstroms.This relationship between deposition time and film thickness ispresented as a guideline, and an actual calibration can be readilyobtained for any particular combination of substrate and treatmentprocedures.

After deposition Is complete, the coated substrate is removed from thedeposition solution and rinsed in demineralized water, but not dried.

At this point, the silver is present as a metallic deposit upon thetreated surface of the substrate. It could be used in this condition forsome applications, but is preferably stabilized to avoid chemical andphysical changes during use. The metallic silver deposit is stabilizedby exposing the surface to a stabilization solution. This solution isprepared by dissolving at least about 0.001, preferably from about 0.001to about 0.1, and most preferably from about 0.02 to about 0.05, gramsper liter of a salt of a platinum group metal (such as platinum,palladium, rhodium, iridium, ruthenium, and osmium) or gold, preferablya platinum salt, into dilute hydrochloric acid. The dilute acid ispreferably prepared by boiling conventional concentrated hydrochloricacid to remove water, and then diluting the acid with demineralizedwater to a pH of from about 3.0 to about 4.8. The stabilization solutionshould be used within 8 hours of preparation, and is preferablydiscarded after 2-3 uses. The stabilization solution is contacted to thesurface for at least about 5 seconds at ambient temperature, andpreferably for 1-20 minutes at ambient temperature.

After the stabilization treatment, the substrate surface is rinsed indemineralized water and dried. It is then ready for use, having anadherent silver coating that is uniformly of a thickness determined bythe deposition time. Large numbers of pieces can be coated at a timeusing this approach, and the pieces may be of irregular size and shape.Coating is accomplished on the inside of even small bores if thesolutions can be contacted to the inside walls. In some instances, itmay be necessary to force the various solutions through the small boresto achieve wetting and reaction. Using the technique of the invention,silver has been coated into bores as small as 0.002 millimeters indiameter.

The preceding processing treatment is sensitive to impurities in thesolutions. It is therefore preferred that reagent grade chemicals anddemineralized (deionized) water be used in all procedures.

The following examples are presented as illustrative of the process ofthe invention and its results, and should not be taken as limiting ofthe invention in any respect.

EXAMPLE 1

A uniform layer of silver was deposited on the surface of apolycarbonate substrate. The polycarbonate was first immersed in a 5percent sodium hydroxide cleaning solution at 40° C. for 10 minutes,followed by a rinsing in demineralized water. The substrate was thenactivated by immersion in solution of 0.05 grams per liter of stannousfluoride having a pH of 2.5, at 25° C. for 15 minutes, and rinsed indemineralized water. The surface was then plated with silver byImmersion in a freshly prepared deposition solution containing 0.015grams per liter silver nitrate, 0.05 milliliters per liter ammonia, 0.05grams per liter sodium citrate, 0.05 grams per liter invertose, and0.001 milliliters per liter formaldehyde. The deposition step wasperformed in a dark room at ambient temperature. In one instance, thesubstrate was immersed for 2 minutes, yielding a silver layer about 500Angstroms thick. In another instance, the substrate was immersed for 5minutes, yielding a silver layer about 1200 Angstroms thick. In eachcase, the substrate was rinsed in demineralized water after depositionwas complete. In each case, the deposited silver layer was stabilized bydipping the surface into a stabilization solution of 0.06 grams perliter of gold chloride, at 35° C. for 30 seconds. The stabilizedsubstrate was then rinsed in demineralized water and dried by a jet ofcompressed air.

EXAMPLE 2-7

The process of claim 1 was repeated, using, as the respectivesubstrates, synthetic rubber, polyester, polyurethane,polyvinylchloride, polystyrene, and polyamide. Deposition was successfulin each instance.

EXAMPLE 8

An aluminum oxide substrate was cleaned by immersion at 60° C. for 20minutes in a cleaning solution of 25 percent concentration sulfuricacid. During 5 of the 20 minutes, the cleaning solution wasultrasonically agitated. The substrate was rinsed in demineralizedwater. The substrate was then activated by placing it in a freshlyprepared activation solution of 0.2 grams per liter stannous chloride,for 15 minutes at ambient temperature, and then rinsed in demineralizedwater. The substrate was coated with silver by immersing it in the samedeposition solution as described for Example 1, except that the time ofcontact was 20 minutes, and rinsed in demineralized water. The aluminumoxide was then stabilized by immersion in a stabilization solution of0.01 grams per liter of platinum chloride for 1 minute, followed by arinse in demineralized water and drying.

EXAMPLE 9

Another aluminum oxide substrate was cleaned by immersion in a 5 percentsodium hydroxide solution at 60° C. for 20 minutes, followed by rinsingin demineralized water containing hydrochloric acid with a pH of 1.5.The substrate was activated by immersing it in a solution of 0.2 gramsper liter stannous chloride for 15 minutes at ambient temperature,followed by rinsing in demineralized water. A silver layer was depositedby immersing the substrate, in darkness at 15° C. for 90 seconds, in adeposition solution of 0.01 grams per liter silver nitrate, 0.05milliliters per liter ammonia, and 0.08 grams per liter sodium citrate.The substrate was washed in demineralized water, and dried. Nostabilization treatment was performed for this example. The resultingsilver layer was about 350 Angstroms thick.

EXAMPLE 10

A polyimide substrate was cleaned for 5 minutes in acetone at ambienttemperature, and then rinsed in demineralized water. It was immersed ina 0.15 grams per liter stannous fluoride activation solution alsocontaining 10 percent acetone, at a temperature of 30° C. for 10minutes, followed by rinsing in demineralized water. Silver depositionwas accomplished as in Example 1, for an immersion time of 5 minutes,followed by rinsing in demineralized water. The coated surface wasstabilized in a solution of 0.005 grams per liter of platinum chlorideand 0.005 grams per liter of gold chloride, with sufficient hydrochloricacid added to lower the pH to 4.1. The stabilization treatment was at40° C. for 10 minutes.

EXAMPLE 11

A silver layer was deposited upon the inside and the outside of acatheter made of latex (natural rubber). The latex sheet was cleaned ina cleaning solution containing 1-5 percent of sodium hypochlorite, atambient temperature for 2 minutes, followed by rinsing in demineralizedwater. It was placed into an activating solution of 0.05 grams per literof stannous chloride at ambient temperature for 10 minutes, followed byrinsing in demineralized water. Silver was deposited by placing thelatex sheet into a bath containing 0.01 grams per liter of silvernitrate, 0.10-0.12 grams per liter sodium citrate, and sufficientammonia to achieve a pH of from about 8.5 to about 9.5. The silver layerwas stabilized a solution of 0.1 percent platinum chloride inhydrochloric acid to a pH of about 4.1, for a time of 1 minute atambient temperature.

EXAMPLE 12

A borosilicate glass plate was immersed in a cleaning solution of 0.5percent hydrofluoric acid with 10 percent sulfuric acid for 5 minutes atambient temperature, and rinsed thoroughly in demineralized water. Itwas activated in a 0.01 percent solution of stannous fluoride, andrinsed. It was then dipped into a solution of silver nitrate at 0.05grams per liter, together with 0.02 percent of hydrazine hydratereducing agent and sodium hydroxide and ammonia to a pH value of 8.5,for a time of 10 minutes. After rinsing, the substrate was stabilized inan acid solution of 0.05 grams per liter palladium chloride and dried.

EXAMPLE 13

A substrate of ABS plastic polymer was cleaned in a solution of 350grams per liter of chromic acid and 25 percent sulfuric acid at 67° C.for 5 minutes. After rinsing, the substrate was sprayed with a 0.05grams per liter solution of stannous chloride, and rinsed. Silver wasdeposited from a solution of 0.01 grams per liter silver nitrate, 0.05grams per liter sodium acetate, 0.01 milliliters per liter hydrazinesulfate, and ammonia to a pH value of 9.0, by immersing the substratefor 5 minutes. The silver film was stabilized in a 0.1 percent acidsolution of platinum chloride.

EXAMPLE 14

Eight batches of latex catheters having twenty-five catheters per batchwere coated with silver using the approach of Example 11, and then latertested for antimicrobial activity. All catheters showed increasedresistance to microbial activity as compared with uncoated latexcatheters, indicating that the batch process was successful in attaininga high yield of coated catheters.

EXAMPLE 15

A latex catheter coated with hydrogel was further coated with a silverlayer using the procedure of Example 11.

EXAMPLE 16

Example 11 was repeated, and then a hydrogel layer was coated over thesilver coating using a conventional dipping procedure.

EXAMPLE 17

A polyurethane catheter was coated with silver using the procedure ofExample 1.

EXAMPLE 18

A teflon (polytetrafluoroethylene) coated latex catheter was coated withsilver using the procedure of Example 11, after first etching the tefloncoating in liquid sodium.

EXAMPLE 19

The interior of a polyethylene bottle was coated with silver using thepresent approach of Example 1, to provide a barrier coating.

EXAMPLE 20

The adhesion of silver layers on latex specimens, prepared by thepresent approach of Example 19 and a prior art electrolytic depositionapproach, was evaluated qualitatively and quantitatively by severalapproaches. In one, ultrasonic energy was introduced into the specimen,and increased until the bond between the silver layer and the substratewas weakened to an extent that the silver layer could be removed. Thespecimen prepared by the present approach withstood four times as muchultrasonic energy as the electrolytically coated specimen before thesilver layer could be removed. The interfaces of other specimens wasviewed in an electron microscope. In each case the interfaces containedsmall pores. The pore size for the specimen prepared by the presentapproach was less than 3 Angstroms, while the pore size for theelectrolytically prepared specimen was 15-20 Angstroms. A smaller poresize suggests better interface properties for the specimen prepared bythe present approach.

EXAMPLE 21

A number of catheters of different compositions and coatings wereprepared. The silver-coated catheters were prepared by the process ofthe present invention. A 10 square centimeter area of each catheter wasplaced into a vial containing 5 milliliters of a culture medium. Afterincubation at 37° C. for 48 hours, the extract was diluted with mediumto give final extract concentrations of 5, 25, 50, 75, and 100 percent.Cell monolayers of mouse fibroblast cell line L929 were established inplastic multiwell plates. One milliliter of the cell medium was replacedby the catheter extract from the dilutions. The plates were incubatedfor an additional 48 hours. The extract concentrations were prepared andassessed in triplicate.

One hour before termination of the cultures, 1.0 microcuries of ³H-thymidine was added to each well. The cells were rinsed with ice-cold1.5 percent perchloric acid, 0.7 milliliters of 5 percent perchloricacid was added to each well, and the well was heated to 65° C. for 1hour. After cooling, the fluid was transferred to a scintillation vialand the counts per minute recorded. The results are expressed as themean percentage of control plotted against extract concentration. Theextract concentration which depressed uptake to 50 percent of control,termed IC₅₀, was determined for each material. It is known that thehigher the IC₅₀ value, the least urethral inflammation is produced by acatheter.

The catheter materials and coatings, with the IC₅₀ value for each, are:latex, 21.7; silver-coated latex, 71.2, silver nitrate-coated latex,36.3, silver sulfate-coated latex, 43.8; teflon-coated latex, 55.3;silver-coated teflon, 81.2, silver nitrate-coated teflon, 62.4; silversulfate-coated teflon, 64.9; silicone, no toxicity; silver-coatedsilicone, no toxicity; silver nitrate-coated silicone, 66.4; silversulphate-coated silicone, 75.6. The silver-coated catheter materials aresuperior in IC₅₀ value to uncoated materials and those coated with asilver salt.

EXAMPLE 22

Artificial urine containing P. aeruginosa was circulated throughcatheters made of latex, and latex coated with silver by the approach ofExample 11. After up to 10 hours of circulation, disks of each catheterwere removed for examination in a scanning electron microscope, aftertreatment to made the bacteria visible. The visual examination showedthat initially neither material had bacterial cells. After 10 minutes ofexposure to the artificial urine, the latex specimen showed significantnumbers of bacteria, while the silver coated specimen had no observablebacteria. After 10 hours of exposure, the latex disks were completelyoccluded by adherent bacteria, but there was no colonization on thesilver-coated latex disks.

The present invention provides a method to coat various types andconfigurations of nonconducting substrates with thin layers of silver.These silver layers are beneficially used in medical, opticaltransmission, and barrier applications, among others. Medical devicessuch as access devices, lead devices, implants, gloves, condoms,catheters, and wound dressings may be coated. Bottles are coated toprovide a gas barrier. Transparent nonconductive substrates are coatedwith a thin, invisible film of silver to absorb thermal energy. Thesilver films of the present invention are most advantageously applied inthicknesses of less than that required to be visible to the eye.

The process of the invention thus provides an important improvement tothe art of preparing uniform thin films of silver onto nonconductingsubstrates. The films can be prepared reproducibly in a commercialsetting, using dilute, benign wet chemical solutions. Although aparticular embodiment of the invention has been described in detail forpurposes of illustration, various modifications may be made withoutdeparting from the spirit and scope of the invention. Accordingly, theinvention is not to be limited except as by the appended claims.

What is claimed is:
 1. An article that resists microbial growth preparedby a method which consists essentially of(a) activating at least aportion of the surface area of an article which article is constructedof a nonconducting material selected from the group consisting of latex,polystyrene, polyester, polyvinylchloride, polyurethane, ABS polymers,polycarbonate, polyamide, polytetrafluoroethylene, polyimide andsynthetic rubber; followed by (b) chemically depositing a silver layerof 2-2000 Å by treating said activated surface with an aqueous solutionof at least one salt of silver in the presence of a deposition controlagent, said depositing being conducted for only sufficient time toresult in said 2-2000 Å silver layer; followed by (c) rinsing indemineralized water and drying.
 2. The article of claim 1 which isfurther coated with a hydrogel layer.
 3. The article of claim 1 whereinsaid coating is transparent to the naked eye.
 4. The article of claim 1wherein the thickness of the silver layer is 2-350 Å.
 5. The article ofclaim 4 wherein the thickness of the silver layer is 2-50 Å.
 6. Anarticle that resists microbial growth which is constructed of anonconducting substrate selected from the group consisting of latex,polystyrene, polyester, polyvinylchloride, polyurethane, ABS polymers,polycarbonate, polyamide, polytetrafluoroethylene, polyimide andsynthetic rubber, wherein said nonconducting substrate is activated overat least a portion of its surface and said activated portion is coatedwith a coating consisting essentially of an adhesive layer of silver,wherein said silver layer is 2-2000 Å in thickness, andwherein saidsilver layer is in colloidal form.
 7. The article of claim 6 which isfurther coated with a hydrogel layer.
 8. The article of claim 6 whereinsaid coating is transparent to the naked eye.
 9. The article of claim 6wherein the thickness of the silver layer is 2-350 Å.
 10. The article ofclaim 9 wherein the thickness of the silver layer is 2-50 Å.
 11. Anarticle that resists microbial growth which is constructed of anonconducting substrate selected from the group consisting of latex,polystyrene, polyester, polyvinylchloride, polyurethane, ABS polymers,polycarbonate, polyamide, polytetrafluoroethylene, polyimide andsynthetic rubber, wherein said nonconducting substrate is activated overat least a portion of its surface and said activated portion is coatedwith a coating consisting essentially of an adhesive layer ofsilverwherein said silver layer is 2-2000Å in thickness.
 12. The articleof claim 11 wherein the thickness of the silver layer is 2-350 Å. 13.The article of claim 12 wherein the thickness of the silver layer is2-50 Å.
 14. The article of claim 11 which is further coated with ahydrogel layer.
 15. The article of claim 11 wherein said coating istransparent to the naked eye.
 16. An article that resists microbialgrowth which is constructed of a nonconducting substrate, wherein saidnonconducting substrate is coated over at least a portion of its surfacearea with an adhesive, antimicrobial, biocompatible coating consistingessentially of a layer of silver that has been stabilized by exposure toa solution containing the salt of one or more platinum group metals orgold or a combination thereof, wherein said coating is 2-2000 Å inthickness, andwherein said silver layer is in colloidal form.
 17. Thearticle of claim 16 wherein said nonconducting substrate is selectedfrom the group consisting of latex, polystyrene, polyester,polyvinylchloride, polyurethane, ABS polymers, polycarbonate, polyamide,polytetrafluoroethylene, polyimide and synthetic rubber.
 18. The articleof claim 17 wherein the nonconducting substrate is latex.
 19. Thearticle of claim 16 wherein the platinum group metal is selected fromthe group consisting of platinum, palladium, rhodium, iridium, rutheniumand osmium.
 20. The article of claim 19 wherein said platinum groupmetal is palladium.
 21. The article of claim 16 wherein the totalsurface area of said nonconducting substrate is coated.
 22. The articleof claim 16 which is further coated with a hydrogel layer.
 23. thearticle of claim 16 wherein said coating is transparent to the humaneye.
 24. The article of claim 16 wherein the thickness of the silverlayer is 2-350 Å.
 25. The article of claim 16 wherein the thickness ofthe silver layer is 2-50 Å.
 26. An article that resists microbial growthwhich is constructed of a nonconducting substrate, wherein saidnonconducting substrate is coated over at least a portion of its surfacearea with an adhesive, antimicrobial, biocompatible coating consistingessentially of a layer of silver that has been stabilized by exposure toa solution containing the salt of one or more platinum group metals orgold or combination thereof,wherein said coating is 2-2000 Å inthickness.
 27. The article of claim 26 wherein said nonconductingsubstrate is selected from the group consisting of latex, polystyrene,polyester, polyvinylchloride, polyurethane, ABS polymers, polycarbonate,polyamide, polytetrafluoroethylene, polyimide and synthetic rubber. 28.The article of claim 26 wherein the nonconducting substrate is latex.29. The article of claim 26 wherein the platinum group metal is selectedfrom the group consisting of platinum, palladium, rhodium, iridium,ruthenium and osmium.
 30. The article of claim 26 wherein said platinumgroup metal is palladium.
 31. The article of claim 26 wherein the totalsurface area of said nonconducting substrate is coated.
 32. The articleof claim 26 which is further coated with a hydrogel layer.
 33. Thearticle of claim 26 wherein said coating is transparent to the nakedeye.
 34. The article of claim 26 wherein the thickness of the silverlayer is 2-350 Å.
 35. The article of claim 26 wherein the thickness ofthe silver layer is 2-50 Å.